The present invention relates to a stimulable phosphor sheet employable in a radiation image recording and reproducing method utilizing stimulated emission from a stimulable phosphor.
As a method replacing a conventional radiography using a combination of a radiographic film and radiographic intensifying screens, a radiation image recording and reproducing method utilizing a stimulable phosphor was proposed and has been practically employed. The method employs a radiation image storage panel comprising a stimulable phosphor layer (i.e., stimulable phosphor sheet) provided on a support, and the procedure of the method comprises the steps of causing the stimulable phosphor in the phosphor sheet to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with stimulating rays to release the radiation energy stored in the phosphor as light emission (i.e., stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals. The stimulable phosphore sheet thus processed is subjected to a step for erasing a radiation image remaining therein, and then stored for the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly employed.
Generally, a substrate film and a protective cover film are provided on the top and bottom surfaces of the stimulable phosphor sheet, respectively. The stimulable phosphor sheet generally comprises a binder and stimulable phosphor particles dispersed therein, but it may consist of agglomerated phosphor without binder. The phosphor sheet containing no binder can be formed by deposition process or sintering process. Further, the sheet comprising agglomerated phosphor soaked with a polymer is also known. In the aforementioned method, any types of the stimulable phosphor sheets are employable.
The radiation image recorded in the stimulable phosphor sheet is generally read by the steps of applying stimulating rays onto the front surface side (phosphor layer side) of the phosphor sheet, collecting light emitted by the phosphor particles by means of a light-collecting means from the same side, and photoelectrically converting the light into image signals. A system for reading the image from one side of the panel in this manner is referred to as xe2x80x9csingle-side reading systemxe2x80x9d. However, there is a case that the light emitted by the phosphor particles should be collected from both sides (i.e., front and the back surface sides) of the phosphor sheet. For instance, there is a case that the emitted light is desired to be collected as much as possible. There also is a case that the radiation image recorded in the phosphor layer varies along the depth of the layer, and that it is desired to detect the variation. A system for reading the image from both sides of the phosphor sheet is referred to as xe2x80x9cdouble-side reading systemxe2x80x9d.
The radiation image recording and reproducing method is often used in radiography for medical diagnosis. In that case, it is especially desired to reproduce a radiation image of high quality (particularly, high sharpness for high resolution) by applying a small dose of radiation. Therefore, the stimulable phosphor sheet is required to have a high sensitivity and to provide an image of high quality.
The sharpness of radiation image is mainly affected by diffusion or scattering of the stimulating rays in the phosphor sheet or layer. The procedure for reading the latent image comprises the steps of sequentially scanning a beam of the stimulating rays on the surface of the phosphor sheet to induce the stimulated emission, and successively collecting and detecting the emission. If the stimulating rays diffuse or scatter (horizontally in particular) in the sheet, it excites the phosphor not only at the target spot but also in its periphery. Consequently, the stimulated emission emitted from the target position is collected together with that from the periphery. Such contamination of the emissions impairs the sharpness of the resultant image.
For avoiding the diffusion or scattering of the stimulating rays, it has been proposed to divide the plane of the stimulable phosphor sheet into small sections with partitions reflecting the stimulating ray.
Japanese Patent Provisional Publication No. 59-202100 discloses a stimulable phosphor sheet of a honeycomb structure consisting of many small cells filled with a stimulable phosphor. The phosphor sheet is composed of a substrate and a stimulable phosphor layer, and the honey-comb structure sectioned with a partition is further provided on the phosphor layer.
Japanese Patent Provisional Publication No. 62-36599 discloses a stimulable phosphor sheet employing a support provided with many hollows regularly arranged on one surface. The hollows are filled with stimulable phosphor, and the ratio of depth to diameter of each hollow is 3.5 or more.
Japanese Patent Provisional Publication No. 5-512636 discloses a process for preparing pixel phosphors with a mold.
Japanese Patent Provisional Publication No. 2-129600 discloses a storage panel employing a support plate having many holes vertically bored and filled with a stimulable phosphor.
Further, Japanese Patent Provisional Publication No. 2-280100 discloses a stimulable phosphor sheet employing a substrate having a honey-comb micro-structure filled with a stimulable phosphor.
In each aforementioned known phosphor sheet employing a support or substrate provided with many holes or hollows incorporated with a phosphor, a part of the support or substrate serves as a partition preventing the simulating rays from diffusion. The stimulable phosphor sheet of this type, therefore, is useful for improving quality (particularly, sharpness) of the resulting radiation image. On the other hand, since the partition of support material partly occupies the phosphor layer, the amount of the phosphor incorporated in a unit volume of the layer is often too small to absorb enough amount of radiation. Consequently, the partition lowers the sensitivity of the stimulable phosphor sheet. Although the sensitivity can be enhanced by thickening the phosphor layer, a thick phosphor layer generally impairs the sharpness.
In radiography for medical diagnosis, the use of a stimulable phosphor sheet of high sensitivity can reduce a dose of radiation to be applied to a patient. Therefore, it is needed to provide a stimulable phosphor sheet giving an image of higher sharpness with higher sensitivity.
The present invention resides in a stimulable phosphor sheet for radiation image recording and reproducing method comprising the steps of recording a radiation image as a latent image, irradiating the latent image with stimulating rays to release stimulated emission, collecting the stimulated emission, and electrically processing the collected emission to reproduce the radiation image, comprising at least two partitioned stimulable phosphor films laminated one on another, the partitioned stimulable phosphor film comprising plural partitions that divide the stimulable phosphor film on a plane thereof to give plural stripe sections, and a stimulable phosphor layer placed in each stripe section, in such manner that the partitions of one stimulable phosphor film are arranged to cross the partitions of another stimulable phosphor film.
In the stimulable phosphor sheet of the invention, the partitions of one stimulable phosphor film are preferably arranged to perpendicularly cross the partitions of another stimulable phosphor film, or to cross the partitions of another stimulable phosphor film at an angle of 45xc2x0 or 60xc2x0.
In the invention, the stimulable phosphor sheet preferably comprises 2 to 100 stimulable phosphor films, more preferably comprises 2 to 10 stimulable phosphor films, and preferably has a thickness in the range of 50 xcexcm to 1,500 xcexcm. The stimulable phosphor film preferably has a thickness in the range of 10 xcexcm to 1,000 xcexcm, more preferably 50 xcexcm to 200 xcexcm. The stimulable phosphor film laminated on another stimulable phosphor film preferably has a thickness smaller than that of the latter phosphor film, so as to improve an image quality of the radiation image reproduced in the radiation image reproducing procedure.
The stimulable phosphor films of the invention preferably has the partition comprising low light-absorbing fine particles and a binder. In particular, the low light-absorbing fine particles comprises metal oxide.
The stimulable phosphor layer of the invention preferably comprises stimulable phosphor particles and a polymer binder.
In the stimulable phosphor sheet of the invention, the laminate of the stimulable phosphor films are placed between a transparent cover film and a transparent substrate film or between a transparent cover film and a light-reflective substrate film.
The above-mentioned stimulable phosphor sheet of the invention is preferably produced by a process which comprises the steps of:
producing plural stimulable phosphor films and plural partition films;
laminating the stimulable phosphor films and the partition films alternately, to produce a laminate block;
slicing the laminate block perpendicularly to the plane of the phosphor film, so as to give plural partitioned stimulable phosphor films;
laminating the plural partitioned stimulable phosphor films in such manner that the partitions of one stimulable phosphor film are arranged to cross the partitions of another stimulable phosphor film; and
heating the laminate under pressure.